Commentary from Rabbi Yehuda Ashlag
From the Sefer: The Shadow of the Ladder
This is the way of the Torah: You should eat bread with salt. You should drink water in strict measure. You should sleep upon the earth and live a life of suffering and you should labor in the Torah. If you act this way, you will be happy and it will be happy for you. You will be happy in this world and it will be well for you in the world-to-come.